It could be a number of things but I suspect that some ports aren't open. What sort of router/firewall are you using and what ports have you forwarded to the IP of the computer running 3CX? Have you followed the information available on the 3CX site?
This is a typical firewall / nat problem. Your server could be on a public IP but you could still be passing via a NAT device for the phone or even one located at the ISP that you are not aware of!
The PBX is set to deliver audio in the PSTN settings.. The default gateway for both phones are set to the address of the NIC in the 3CX server for that subnet.